A mutagenized subline of Balb/c 3T3 cells in culture has been obtained (designated THO) which is resistant to thioguanine and ouabain, permitting isolation of cell hybrids between it and normal or transformed cells in selective medium. The regulation of the expression of transformation will be studied in cell hybrids between THO and viral or chemically transformed murine cells to answer the following questions. (1) Is the transformed or untransformed phenotype dominant? (2) Is transformation suppressed in hybrids between cell lines transformed by different oncogenic agents? (3) Can prototype cell lines transformed in different properties be identified by complementation analysis? Results obtained thus far demonstrate that the transformed phenotype is dominant in cell hybrids between SV40-transformed and normal mouse cells in regard to colony morphology and serum dependence of DNA synthesis.